1. Field of the Invention
The invention pertains to the field of variable valve timing mechanisms. More particularly, the invention pertains to an actuator and control method for a variable valve timing mechanism.
2. Description of Related Art
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission devices, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative.
However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension. For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must be very rigid when the chain tightens.
To accomplish this result, a hydraulic tensioner 1, as shown in prior art FIG. 1, typically comprises a rod or cylinder as a piston 2, which is biased in the direction of the chain by a tensioner spring 3. The piston 2 is housed within a cylindrical housing 5, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the housing contains a pressure chamber 4 in connection with a reservoir or exterior source of hydraulic fluid pressure. The pressure chamber 4 is typically formed between the housing 5 and the piston 2, and it expands or contracts when the piston 2 moves within the housing 5.
Typically, valves are employed to regulate the flow of fluid into and out of the pressure chamber. For instance, an inlet check valve 6 typically includes a ball-check valve that opens to permit fluid flow in to the pressure chamber 4 when the pressure inside the chamber has decreased as a result of outward movement of the piston 2. When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber. The closing of the inlet check valve 6 prevents the piston chamber from contracting, which in turn prevents the piston from retracting, achieving a so-called “no-return” function.
Many tensioners also employ a pressure relief mechanism that allows fluid to exit the pressure chamber when the pressure in the chamber is high, thus allowing the piston to retract in response to rapid increases in chain tension. In some tensioners, the pressure relief mechanism is a spring biased check valve. The check valve opens when the pressure exceeds a certain pressure point. Some tensioners may employ a valve which performs both the inlet check function as well as the pressure relief function.
Other mechanisms employ a restricted path through which fluid may exit the fluid chamber, such that the volume of flow exiting the fluid chamber is minimal unless the pressure in the fluid chamber is great. For instance, a restricted path may be provided through the clearance between the piston and bore, through a vent tube in the protruding end of the piston, or through a vent member between the fluid chamber and the fluid reservoir.
A hydraulic tensioner as used with a tensioner arm or shoe is shown in Simpson et al., U.S. Pat. No. 5,967,921, incorporated herein by reference. Hydraulic chain tensioners typically have a plunger slidably fitted into a chamber and biased outward by a spring to provide tension to the chain. A lever, arm or shoe is often used at the end of the plunger to assist in the tensioning of the chain. The hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing. The plunger is moved outward against the arm by the combined efforts of the hydraulic pressure and the spring force.
When the plunger tends to move in a reverse direction (inward) away from the chain, typically a check valve is provided to restrict the flow of fluid from the chamber. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements of the plunger are easy in one direction (outward) but difficult in the reverse direction.
Blade tensioners are tensioners that are commonly used to control a chain or belt where load fluctuations are not so severe as to over flex the spring or springs. A ratchet with backlash is added to tensioners to limit the effective backward or untensioned travel of a tensioning device.
Prior art FIG. 2 shows an example of a blade tensioner. The conventional blade tensioner 110 includes a blade shoe 111 made of resin having a curved chain sliding face and numerous blade springs 121, preferably made of metallic material. The blade springs 121 are arranged in layers on the opposite side of the blade shoe 111 from the chain sliding face, and provide spring force to the blade shoe 111. The ends of each spring-shaped blade spring 121 are inserted in the indented portions 114 and 115, which are formed in the distal portion 112 and proximal portion 113 of the blade shoe 111, respectively.
A bracket 117 is provided for mounting the blade tensioner 110 in an engine. Holes 118 and 119 are formed in the bracket 117, and mounting bolts are inserted into these holes 118 and 119. A sliding face 116 contacts the distal portion of the blade shoe 111 and permits sliding. The slide face 116 is formed on the distal portion of the bracket 117. A pin 120 supports the proximal portion 113 of the blade shoe 111 so that it may move in either direction. The pin 120 is secured on the bracket 117.
FIG. 3 shows a chain tensioning device that has a pair of arms 202, 203 which are joined by a pivot 204. The arms 202, 203 are urged apart so that arm 203 applies tensioning force to a chain (not shown) by means of a spring 206 loaded cam block 205. To prevent collapse of arm 203 during load reversals of the chain, a catch disc 209 and rod are arranged to prevent return movement of the spring loaded cam block 205.
FIG. 4 shows an example of a tensioner that uses a ratchet device. The ratchet tensioner 301 comprises a tensioner housing 307 having a hole 312 for receiving a plunger 308 and a ratchet pawl 317 pivoted by a shaft 316 to the tensioner housing 307 and biased by a ratchet spring 318. The plunger 308 has teeth on one outer side that engage the ratchet pawl 317. The plunger 308 is biased out of the hole 312 to contact the tension lever 310 by fluid in the hollow section 313 and by the plunger spring 314. The tensioner lever 310 pivots on support shaft 309 and has a shoe surface 311 that contacts and applies tension to the slack side of the timing chain 306 wrapped around the camshaft 304 and its sprocket 305 and the crankshaft 302 and its sprocket 303. The plunger's 308 movement in and out of the hole 312 is limited by its teeth and the ratchet pawl 317 that engage them.
FIG. 5 shows a tensioning device of U.S. Pat. No. 6,599,209. The tensioning device 421 includes a one piece supporting body 422 with interconnecting sections 424 in the shape of an open honeycomb joined to a tensioning track 423. The tensioning track 423 is elastically supported by coil spring 25. Mounting holes 420 are used to fixedly mount the tensioner 421. In alternate embodiments shown in prior art FIGS. 6a and 6b, a pair of tensioning tracks 406 are joined by rib like interconnecting sections 407 to form one piece. Interconnecting sections 407 are elastically deformable and have angled sections 408. Between the tensioner tracks 406 and the supporting body (not shown) are either a single leaf spring 409 or a leaf spring packet 409a. One of disadvantages of the tensioning device of U.S. Pat. No. 6,599,209 is the spring only tensions the center of the tensioning track and not along the entire face of the tensioner. Another disadvantage is the assembly required to place the spring within the supporting body, since the tensioner is one piece.
FIG. 7 shows the tensioner of U.S. Pat. No. 6,849,015. A shoe 533 is attached to an arm 534 and is positioned outside of the slack side of the chain. The tensioner arm 534 applies tension to the chain when moved in the direction marked by arrow 539. The arm 534 and the shoe 533 are attached by a fixed pin 538 to a tensioner pivot arm 531. The tensioner pivot arm 531 has a pivot 532. Tensioning occurs when a force is applied at point 535 in the direction indicated by arrow 536. Excessive reverse rotation of the tensioner pivot arm 531 is limited by a ratchet feature.